1. Field of the Invention
The present invention relates to a steering system for a vehicle with steerable front and rear wheels.
2. Description of Relevant Art
There have been known various conventional steering systems for road vehicles, such as automobiles, having steerable front and rear wheels. Some of such steering systems permit a specific steering operation mode in which the front wheels and the rear wheels may be selectively steered in the same direction and in opposite directions, depending on certain running conditions of the vehicle. For example, Furukawa et al U.S. Pat. No. 4,313,514 discloses steering systems for steering the rear wheels in the same direction as the front wheels when the vehicle speed is relatively high and in the direction opposite to that of the front wheels when the vehicle speed is relatively low, so as to provide favourable steering characteristics of the vehicle. Sano et al U.S. Pat. No. 4,506,899 discloses another steering system, which steers the rear wheels in the same direction as the front wheels when the amount of rotation of the steering wheel from its neutral position is relatively small and in the opposite direction when such amount of rotation is relatively large.
When a vehicle with such steering system makes a turn with the front and rear wheels being steered in the same direction, the turning radius of the vehicle will be so large that the vehicle proceeds obliquely with its movement being nearly a translation. This results into a small lateral acceleration of the vehicle caused by a small centrifugal force during a turn.
Generally, lateral accelerations of a vehicle considerably affect the returnability of the vehicle. The returnability is the tendency of the road wheels to return to the straight-ahead position, or, the tendency of the steering wheel to return its neutral position. The lateral acceleration is caused by the road resistance applied to the tires in lateral direction (sometimes called side force to the tires), which is transmitted through the steering linkage to the steering wheel as a torque to urge the steering wheel to its neutral position. The torque is perceived by the driver as a returning torque of the steering wheel. Thus, the lateral acceleration has direct effects over the returnability, though there are various other factors which have effects over it. In almost every road vehicle, the returning torque occurs on their steering wheels and substantially affects the operation feelings of the steering wheel.
In a conventional vehicle having a steering system of the aforementioned type, however, it is difficult to provide optimum operation feelings of the steering wheel because small lateral accelerations in a turn, under the condition that the front and rear wheels are steered in the same direction, often result in an insufficient returning torque of the steering wheel.